Climate change 'not reason for extreme winters'

Global warming tends to reduce temperature variability, says a new study showing the recent cold winters seen in the US are not a result of climate change.

Scientists at ETH Zurich and the California Institute of Technology used climate simulations and theoretical arguments to show that in most places, the range of temperature fluctuations will decrease as the climate warms.

Not only will cold snaps become rarer, their frequency will also be reduced.

However, the work does not rule out temperature extremes and in fact warns of more extreme warm periods in the future.

Pictures of icy, snow-covered cities around the world have raised the question whether climate change could be responsible for these extreme events.

The existing scientific explanation has been that amplified warming of the Arctic relative to lower latitudes in recent decades has weakened the polar jet stream driven by temperature differences between the warm tropics and cold polar regions.

A weaker jet stream leads to greater fluctuations in temperature in mid-latitudes including the cold snaps.

Scientists led by Tapio Schneider, professor of climate dynamics at ETH Zurich, have instead argued that the range of temperature fluctuations will decrease with rising mercury. Cold events will be rarer and infrequent.

The study appears in the Journal of Climate.

Higher latitudes are indeed warming faster than lower ones, and the temperature difference between the equator and the poles is decreasing.

As the temperature difference reduces, the air masses will eventually have the same temperature.

Using a highly simplified climate model, the team examined various climate scenarios to verify the above theory.

It showed that the temperature variability in mid-latitudes decreases as the temperature difference between the poles and the equator diminishes.

Climate model simulations by the Intergovernmental Panel on Climate Change (IPCC) have also showed similar results under global warming, indicating lower temperature variability, especially in winter.

As the ETH work was limited to temperature trends, the team noted that extreme events, such as storms with heavy rain or snowfall, can still become more common as the climate warms, as other studies have shown.